1. Field of the Invention
The invention relates to a variable reluctance pickup for steel string musical instruments in which the vibrating strings cause variations of reluctance in a magnetic circuit generating electrical signals which, upon electronic amplification, are suitable for driving acoustic speaker systems.
2. Description of the Prior Art
Generally, variable reluctance pickups for steel string musical instruments comprise an arrangement of magnets and magnetically susceptible materials which establish a magnetic circuit in combination with the playing strings. As the strings vibrate, the changes in their position affect the reluctance and magnetic flux of the magnetic circuit. A sensing coil is inductively linked to the magnetic circuit for converting the variations in magnetic flux into a corresponding electrical signal. The electrical signals from the sensing coils is amplified electronically and fed into an acoustic speaker system for producing musical sounds.
There are many different configurations of the basic elements of variable reluctance pickup systems for steel string instruments. For example, U.S. Pat. No. 2,235,983 (Demuth) describes the basic elements of a magnetic pickup suitable for pianos and the like. U.S. Pat. No. 3,066,567 (Kelly) describes a magnetic pickup system having a single, permanent magnetic element with a plurality of pedestals to provide a specific pickup zone for a given instrument string in combination with a single sensing coil. U.S. Pat. No. 3,483,303 (Warner) describes a variable reluctance transducer pickup system for steel string musical instruments in which an attempt is made to isolate the magnetic circuits formed by adjacent strings so as to minimize "cross-talk" between the various strings. U.S. Pat. No. 3,571,483 (Davidson) describes a variable reluctance pickup system having a plurality of isolated magnetic circuits, each specifically designed to be substantially insensitive to the plane of string vibration. Finally, U.S. Pat. No. 3,715,446 (Kozinski) describes a magnetic pickup system having a balanced coil assembly for each string wherein each assembly includes a bar magnet supporting two circular pole pieces and two sensing coils disposed around the pole pieces.
Before discussing the disadvantages of prior art, variable reluctance pickup systems, it is instructive to review the fundamental properties of string instruments which give them their characteristic tones.
Basically, the tone of a plucked or a struck string instrument is judged by the richness and complexity of the acoustic output in the "attack" or beginning portion of a note. In acoustic string instruments, the bridge structure constrains the motion of the soundboard such that those components of string motion which are perpendicular to the plane of the soundboard are well amplified, while those components of the string motion which are parallel to the plane of the soundboard are not. The path described by any arbitrarily small segment of a smoothly released, plucked string is a precessing elliptical orbit of decreasing radius which rotates about the quiescent position of the string. Accordingly, the asymmetrical amplification of string motion provided by the bridge of an acoustic instrument yields a rich, full and complex tone of continuously varying, harmonic content. The richness and complexity of tones produced by acoustic string instruments are the primary criterion of judging the quality of such instruments.
In addition, the preferential or asymmetrical amplification provided by the bridge structure in acoustic string instruments enhances the expressive ability of the instrument. Specifically, the musician can control the initial motion of the string by plucking either parallel to the soundboard for a "thin or nasal" tone or perpendicular to the soundboard for a "full or rich" tone.
Steel string guitars and other similar instruments have a particular capability which distinguishes them from most other Western musical instruments. This capability is referred to as "bending". "Bending" is accomplished after a string is fretted and plucked by moving the fretting finger with the string across the fingerboard, stretching the string. The stretching of the string during "bending" can raise the pitch of the note by as much as seven semi-tones, a factor which greatly enhances the expressive capability of the instrument. However, "bending" a note also results in a large displacement of the string from its normal vibrating zone about the quiescent string position.
For variable reluctance pickup systems to have good tone (by acoustic instrument standards), it must be highly asymmetrical in converting string motion to electrical signal output. Further, such pickup systems have a capability for high-frequency response in order to preserve the richness and fullness of the varying harmonics in the "attack" portion of a note. Finally, for steel string guitars and similar instruments, the pickup systems must be insensitive to string displacement due to "bending".
The prior art variable reluctance pickup systems are characterized by separate pole tip and/or pole pieces for each string. Each pole tip and/or pole piece provides a distinct magnetic field region around the quiescent position of each string. The distinct magnetic field regions of prior pickup systems render them relatively insensitive to the plane of vibration of the particular string.
For example, pickup systems with circular pole pieces provide a magnetic field having the form of a symmetrical sinusoidal shell and a string vibrating within such a magnetic field will generate approximately equal magnitude electrical signals for string vibrations both parallel and perpendicular to the string plane.
Another disadvantage of the prior art variable reluctance pickup systems relates to their sensitivity to "bending". Specifically, the magnetic field drops off between the individual pole tip and/or pole pieces. Accordingly, the pickups will not uniformly sense a string vibration as the string is displaced from its normal vibrating position during a "bending" motion.
Prior art variable reluctance pickup systems having a single coil for sensing variations of the magnetic circuits have very poor high-frequency responses. Specifically, the impedance of a sensing coil in a magnetic circuit increases with increasing frequency up to a maximum at a resonant frequency whereupon the impedance of the coil decreases. Below the resonant frequency, the impedance of the coil is dominated by inductive effects. In explanation, the resulting variations in magnetic flux due to string vibrations induce an electrical signal in the coil which, in turn, creates another magnetic field which "bucks" or opposes the variations in flux induced by the string (Lenz' Law). This effect "impedes" the signal and increases with increasing frequency. Above the resonant frequency, the impedance is influenced by the capacitive effects between turns of the coil and between layers in the coil winding, i.e., the changing current in one turn of the coil influences current in neighboring turns of the coil. This effect becomes larger with increasing frequencies such that the coil behaves as a capacitive reactance with turn-to-turn capacitive leakage to ground. Accordingly, the output signal from the sensing coil falls off rapidly above the self-resonant frequency. Both the inductances and the cpacitance of a sensing coil vary linearly with the mean radius of the coil. The mean radii in single-coil embodiments of prior art variable reluctance pickups are large. Hence, the "attack" portion of a note is not reproduced accurately.